This is a chapter from Connected Geometry—a high-school geometry curriculum published by Everyday Learning Corporation. See below for links to other chapters from the curriculum.
Connected Geometry was written by the staff at EDC (Education Development Center) under a grant from the National Science Foundation (MDR92-52952). Its primary developers were Nancy Antonellis, Al Cuoco, Pamela Frorer, E. Paul Goldenberg, Jack Janssen, Michelle Manes, June Mark, and Daniel Scher. The curriculum was designed around mathematical “habits of mind.” Such habits include: visualizing, using precise language to describe and analyze, tinkering with problems, mixing deduction with experimentation, reasoning by continuity, looking for invariants, and seeking logical connections.